Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate.

In geological terms, the process of mineral hydration is known as retrograde alteration and is a process occurring in retrograde metamorphism. It commonly accompanies metasomatism and is often a feature of wall rock alteration around ore bodies. Hydration of minerals occurs generally in concert with hydrothermal circulation which may be driven by tectonic or igneous activity.

Mineral hydration is also a process in the regolith that results in conversion of silicate minerals into clay minerals.[citation needed]

Some mineral structures, for example, montmorillonite, are capable of including a variable amount of water without significant change to the mineral structure.[citation needed]

Hydration is the mechanism by which hydraulic binders such as Portland cement develop strength. A hydraulic binder is a material that can set and harden submerged in water by forming insoluble products in a hydration reaction. The term hydraulicity or hydraulic activity is indicative of the chemical affinity of the hydration reaction.[2]

^ abcdefghRivkin, A.S.; Howell, E.S.; Vilas, F.; Lebofsky, L.A. (2002). "Hydrated Minerals on Asteroids: The Astronomical Record"(PDF). Asteroids III. ISBN9780816522811. Retrieved 2018-03-10. Hydrated minerals include both silicates and nonsilicates in the scope of this review. Phyllosilicates (or “clay minerals”) are commonly found on Earth as weathering products of rocks or in hydrothermal systems. Nonsilicate hydrated minerals include such species as the oxides brucite and goethite, the carbonate hydromagnesite, and the sulfide tochilinite, each of which is known in the meteorite collection (Rubin, 1996). Although a full discussion of the petrogenesis and classification of hydrated minerals is beyond the scope of this paper, we note that formation of hydrated minerals, particularly clay minerals, occurs rapidly and easily in environments where anhydrous rock and water are together.